The autonomously replicating parvoviruses, which include canine parvovirus, mink enteritis virus, feline panleukopenia virus, and other porcine, bovine, rodent, and human parvoviruses, are among the smallest known DNA viruses. Infectious virions are spherical, non-enveloped particles 20-25 nm in diameter that contain three species of structural polypeptide, VP1, VP2', and VP2, with overlapping amino acid sequences. The genome of these viruses consists of a predominantly single-stranded, linear DNA molecule with a molecular weight of 1.35-1.70.times.10.sup.6 (approximately 5000 nucleotides in length). This DNA encodes a noncapsid protein (NTCVP1) in the 3' (left-hand) portion of the genome, as well as the major capsid proteins, which are encoded by overlapping regions of viral DNA in the 5' (right-hand) portion of the genome.
Canine parvovirus (CPV) was first recognized as the etiological agent responsible for a series of disease syndromes in dogs in 1978. There is no serological evidence prior to 1978 to indicate existence of the virus before that time. It is believed that CPV arose from a single source around that time. The virus has rapidly spread to pandemic proportions. CPV is genetically and antigenically related to feline panleukopenia virus (FPV), mink enteritis (MEV) and other parvoviruses which cause disease in animals. It is believed that CPV is a host range variant (mutant) of one of these viruses, most likely FPV, since FPV modified live vaccines were and are widely used.
There are two types of parvovirus vaccines currently in use: a modified live virus vaccine and a killed whole virus vaccine. While effective these vaccine preparations have several drawbacks. Modified live virus has the potential for reversion to a virulent form. In addition, continuous use of these live virus vaccines presents the risk that the virus will mutate to develop a wider host range. Further, the ability of modified live virus vaccines to induce protection can be inhibited by maternal antibodies which prevent replication of the modified virus in newborn animals often the time of greatest susceptibility to disease.
Killed virus vaccines do not revert to virulent forms and they are capable of stimulating immunity in the presence of maternal antibodies. However, killed virus preparations are generally less immunogenic and evoke lower antibody titers than live virus preparations. One reason for this is that the procedures for inactivating the virus cause denaturation of viral protein. In addition, killed viruses do not replicate in the vaccinated animal. In order to compensate for this immunogenic deficiency, high doses of killed virus must be given. But, the administration of high doses of protein may cause complications, such as anaphylatic shock; furthermore it can be expensive to produce large quantities of inactivated virus preparations.
CPV, FPLV, and MEV are very closely related, both genetically and serologically. CPV isolates from nature replicate efficiently in dogs and in cat or dog monolayer cell cultures. FPV isolates grow in cats and in cat monolayer cell cultures, but not in dog monolayer cell cultures. Both viruses replicate to a limited extent in the alternate animal host. As a consequence of the similar antigenic structures of these parvoviruses, vaccination with any one of these viruses can produce cross-species protective immunity; thus, both modified live and killed FPV vaccines have been used to protect dogs against canine parvovirus infection, and modified live or killed CPV vaccines have been used to protect minks from mink enteritis, and should elicit protective immunity to FPLV as well.
It has been previously demonstrated that empty capsids of a murine parvovirus, the Minute Virus of mice (MVM) could be produced by recombinant DNA techniques (Pintel et al. J. Virol. 52, 320 (1984)). In those experiments, recombinant plasmids containing the genomes of both bovine papillomavirus type I and MVM were used to transform mouse C127 cells, a natural permissive host for MVM infection. Cells harboring these plasmids could, however, be selected only by their transformed morphology; the dependence on this morphological change as the selective marker for cells harboring the BPV-derived plasmid limits the host range for the use of these plasmids to those cells which display the transformed phenotype, including mouse cell lines C127 and NIH 3T3.
The purpose of the experiments described in the previous paragraph was to generate superinfectible host cell lines which constitutively express MVM gene products as a tool for the study of MVM gene expression. During the course of analysis of the MVM/BPV transformed cell lines, it was noted that the viral proteins VP1 and VP2' assemble into empty virion particles.